As in "that which could be edited," or "editable" if you will.
"editable" seems to be in-use currently and is growing in popularity. Since it seems to be a relatively recent coining, I'd recommend using it for technical audiences but might avoid using it if I thought my audience would prefer more "established" words.
Also, the Corpus of Contemporary American English shows some usage of the word in the way you'd expect. Most of these uses are computer-related or academic publications.
You could use revisable if you don't like editable.
How about "mutable"?
As others have said, "editable" is technically correct. Depending on the context, you may be able to say it in a different way, to avoid the awkward-sounding "editable":
The box is light gray when the text is editable.
The box is light gray when you can edit the text.
You can edit the text when the box is light gray.
The box is light gray when editing is enabled.